The invention relates to a method for regulating the airflow around a windmill blade, said method comprising a step, whereby a spoiler, mounted on the outer side of the blade, is activated and deactivated in order to create a first and respectively second airflow around the blade.
The invention also relates to a device for use when regulating the airflow around a windmill blade, where said device comprises at least one spoiler, which is made from a flexible material, provided with at least one cavity, and where the cavity in a deactivated form has a first volume and in an activated form has a second volume, and where the volume of the cavity can be changed from the deactivated form to the activated form by leading a fluid to the cavity.
It is known that one has to limit the power output of every mill in high wind, otherwise the mill can be overloaded. The normal methods for limiting the power output are stall regulation and pitch regulation.
With ordinary stall regulation, the blades are fixed on the hub of the mill and can not be turned around their longitudinal axis. The adjusting angle the blades have on the hub is adjusted once and for all during the mounting and commissioning of the mill. The blades are designed so the airflow over them in itself provides a greater air resistance in high wind and thereby limits the power output. Because of this passive use of the aerodynamic characteristics of the blades the regulation is simple and robust under all conditions, with only few peak loads.
Ordinary stall regulation has the disadvantage that the maximum power output depends on the air density and the surface roughness of the blade. There will, therefore, be changes in the power output from summer to winter, and when the blades are dirty. The stall effect also depends highly on the design of the front edge of the blade. Small manufacturing tolerances in the form of the front edges can cause considerable differences in the power output level that a windmill stall regulates at.
With pitch regulation the blades are mounted on bearings on the hub of the mill, so they can be turned around their longitudinal axis. In high wind the adjusting angle is always adjusted away from stall, so the lift is limited to provide the exact desired power output. With the active regulation it is possible to compensate for air density, surface roughness of the blade and influence from manufacturing tolerances.
Pitch regulation has the disadvantage that it demands a relatively complicated, active regulation, which in high wind can be sensitive to turbulence. Therefore, pitch regulation in practice presupposes a special generator with a completely or partly variable number of rotation, so the mill can speed up a bit up in case of wind gusts. Otherwise the active regulation cannot keep up with the variations in the wind, which results in too large peak loads. Pitch regulation also has the disadvantage compared to stall regulation with fixed adjusted blades in that the blades must be mounted rotably on the hub of the mill and, therefore, have to be provided with bearings and actuator systems. These components must be able to transfer great loads, and involve increased service needs.
A newer regulation form is active stall regulation. Here the two normal methods for limiting the power output are combined. Like with pitch regulation there are bearings between the blades and the hub of the mill, so that the entrance angle is adjustable, but the actual limiting of the power output in high wind is effected through stall.
Compared with ordinary stall regulation active stall regulation has the advantage that the maximum power output with certainty can be maintained at the desired level, regardless of air density, possible dirt on the blades and the influence from manufacturing tolerances. Compared with pitch regulation active stall regulation has the advantage that the actual regulation is effected with the use of stall, consequently through a passive use of the aerodynamic characteristics of the blade, so the sensibility to turbulence remains small. It is, therefore, not necessary to use special generators, variable rpm or the like in order to avoid high peak loads.
Nevertheless, active stall regulation has its own disadvantages compared to passive stall regulation with fixed adjusted blades. Like with pitch regulation there is the disadvantage that the blades have to be mounted rotably on the hub of the mill and, therefore, have to be supplied with bearings and actuator systems. These components have to transfer great loads, and involve increased service needs. With active stall regulation the regulation is slower than with pitch regulation, and the demands on the actuator systems are, therefore, smaller, but the complexity is, however, considerably larger than with passive stall regulation.
Apart from the regulation systems based on the turning of whole blades, spoilers on fixed adjusted blades are also known, where the regulation takes places through the spoiler effect, normally completely or partly supported by stalling of the blade. The spoiler can typically be designed as a rail, arranged over the suction side of the blade, and which by extension provides increased air resistance and turbulence, and maybe also trips an actual stall. Such spoilers were used on the windmills erected by FL. Smith in Denmark during the Second World War.
Spoiler systems of this kind normally have the disadvantages that they include mechanical parts far out on the blade. According to experience, is it difficult to maintain a high disposal level on such systems since the operating conditions are very difficult, and since the actuator mechanisms for the spoiler normally are badly suited to withstand the hundreds of millions of exposures that the system is subjected to during normal operation. In addition to this spoilers arranged over the blade surface, also when not activated, normally exhibit a certain constant spoiler effect, which reduces the aerodynamic efficiency of the blade. The external mounting can also result in a significant contribution to the noise.
There are other spoiler systems, which consist of rails or bellows deposited in the blade surface. The effect again depends on increased air resistance and turbulence, but in comparison to external spoilers the mechanism is somewhat better protected. Such spoilers are, among others, used in the early windmills of the WindMatic type in Denmark By being deposited in the blade surface this type of spoilers has the advantage that they not in any noticeable way reduce the aerodynamic efficiency of the blade, when they not are extended.
Deposited spoilers normally have the disadvantage that they demand special blade structures with recesses and cavities. Furthermore, they can normally only with difficulty be carried out at the actual front edge, where the air forces are powerful, but must be arranged further back on the suction side of the profile. Here the operating conditions are better, but the effect of the spoiler is in return more limited and, therefore, the spoiler has too be substantially larger. Noise problems may arise at the separating surfaces between the main blade and the spoiler, and the effect can be uncertain during icing and heavy pollution with dust etc., where extending and depositing can be restricted.
From European patent application EP 0 394 882 A1 a spoiler type is known, which consists of a flexible membrane arranged radially along a windmill blade approximately at the middle of the suction side of the blade, and which by inflating triggers stall on those sections of the blade, which are provided with this spoiler. Spoiler systems of this kind has the disadvantage that the effect of a stall triggered at the middle of the suction side normally will not be sufficient to provide a substantial regulation. In addition there is the disadvantage that this spoiler type presupposes a recessed channel on the blade in an area, where an unbroken and continued structure is needed for strength reasons.
From European patent application EP 0 283 730 A1 a flow body is known, where at least the one side is covered with a membrane, which can change form as a function of a inner pressure media Through inflation this body can assumably change its aerodynamic characteristics to such a high degree that a certain regulation can take place. A membrane system of this kind has the disadvantage that it covers large parts of the blade surface, and that the effect of the regulation can be difficult to control because of the large surfaces.
It is known that the air flow over a windmill blade can be regulated with flow regulating means. Examples of such means are for example described in the Danish utility model application DK 95 00238 W. Here, among other things, vortex generators are illustrated (FIG. 5).